This invention relates to a catalytic oxidation module for a gas turbine engine, and in particular, to a catalytic oxidation tube array module.
Catalytic combustion systems are well known in gas turbine applications to reduce the creation of pollutants in the combustion process. As known, gas turbines include a compressor for compressing air, a combustion stage for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor, and a turbine for expanding the hot gas to extract shaft power. Diffusion flames burning at or near stoichiometric conditions with flame temperatures exceeding 3,000xc2x0 F. dominate the combustion process in many older gas turbine engines. Such combustion will produce a high level of oxides of nitrogen (NOx). Current emissions regulations have greatly reduced the allowable levels of NOx emissions. One technique for reducing NOx emissions is to reduce the combustion temperature to prevent the formation of NO and NO2 gases. One method for reducing combustion temperatures is to provide a lean, premixed fuel to the combustion stage. In a premixed combustion process, fuel and air are premixed in a premixing section of the combustor. The fuel-air mixture is then introduced into a combustion stage where it is burned. Another method for reducing the combustion temperature is to partially oxidize a fuel-air mixture in the presence of a catalytic agent before the fuel-air mixture passes to the combustion stage. In typical catalytic oxidation systems, a cooling means is also provided to control the temperature within the catalytic portion of the system to avoid temperature-induced failure of the catalyst and support structure materials. Cooling in such catalytic oxidation systems can be accomplished by a number of means, including passing a cooling agent over a backside of a catalyst-coated material.
U.S. Pat. No. 6,174,159 describes a catalytic oxidation method and apparatus for a gas turbine utilizing a backside cooled design. Multiple cooling conduits, such as tubes, are coated on the outside diameter with a catalytic material and are supported in a catalytic reactor. A portion of a fuel/oxidant mixture is passed over the catalyst coated cooling conduits and is oxidized, while simultaneously, a portion of the fuel/oxidant enters the multiple cooling conduits and cools the catalyst. The exothermally catalyzed fluid then exits the catalytic oxidation system and is mixed with the cooling fluid outside the system, creating a heated, combustible mixture.
To stabilize combustion of the mixture once the fluids have exited the catalytic oxidation system, it is important that flammabllity, such as flame-holding or premature auto-ignition, are minimized during mixing of the fluids. For example, premature auto-ignition can be prevented by completing the mixing process in a time that is less than the time for auto-ignition. Thus, both mixing time and auto-ignition delay time must be considered as the exothermally catalyzed fluid and the cooling fluid are mixed upon exiting the catalytic oxidation system. Accordingly, the exit portions of catalytic combustion systems have been configured to facilitate mixing of the combustion fluids in a combustion stage after the fluids separately exit the catalytic combustion system. For example, in a catalytic oxidizer module consisting of a number of catalyst coated cooling tubes, flow dynamics and mixing of fluids upon exiting the catalytic combustion system may be enhanced by providing flared tube ends at the downstream exit of the module. In addition, the flared tube ends may be closely packed to provide support for the tubes within the module to provide vibration control.
However, flaring of the tuba ends has many drawbacks. Flaring reduces the wall thickness of the tube in the area of the flare, which may lead to localized premature failure. Flaring of the tube ends also strains the tube material, which may cause cracking or embrittlement in the area of the flare. In a closely packed flared tube end configuration, the tubes are subject to wear (e.g. fretting or fret corrosion) where the flared ends abut. Furthermore, a closely packed flared tube end configuration provides no self-containment of the tubes other than the adjacent tube end points of contact. Yet another problem with a flared end tube configuration is that the exit end of the configuration presents flat surfaces that may provide a mechanism for flame attachment, resulting in premature flammability.
A catalytic oxidation module for a gas turbine engine is described herein as including: a pressure boundary element having an inlet end and an outlet end in fluid communication with a downstream plenum, the pressure boundary element separating a first fluid flow of a combustion mixture from a second fluid flow; a catalytic surface exposed to the first fluid flow between the inlet end and the outlet end; and an opening in the pressure boundary allowing fluid communication between the first and second fluid flows upstream of the outlet end. The pressure boundary element may be a tube, and the opening may be formed in the tube. The pressure boundary element may further include a tubesheet with the opening being formed in the tubesheet.
A gas turbine engine is described herein as including: a compressor for supplying a first and second fluid flow of compressed air; a fuel supply for injecting a combustible fuel into the first fluid flow; a catalytic oxidation module for at least partially combusting the combustible fuel in the first fluid flow and providing at least partial mixing of the first and second fluid flows; a combustion completion chamber receiving the first and second fluid flows from the catalytic oxidation module and producing a hot gas; and a turbine for receiving the hot gas from the combustion completion chamber. The catalytic oxidation module of the gas turbine may further include: a pressure boundary element having an inlet end and having an outlet end in fluid communication with the combustion completion chamber, the pressure boundary element separating the first and second fluid flows along a portion of its length; a catalytic surface exposed to the first fluid flow between the inlet and outlet ends; and an opening in the pressure boundary element allowing fluid communication between the first and second fluid flows upstream of the outlet end.